System for inspecting the surfaces of objects

ABSTRACT

An inspection system optically examines the surfaces of objects to detect surface errors. The system scans image strips and, consequently, a given surface rapidly and with sufficient resolution using a linescan camera and an upstream microscope by aligning the captured lines.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTApplication No. PCT/DE01/01442 filed on 12 Apr. 2001 and GermanApplication No. 100 19 486.9 filed on 19 Apr. 2000, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a system and a method for optically testingsurfaces for defects contained therein.

Quality control plays an important role in the automatic fabrication ofindustrial parts, in particular of semiconductor products and, inparticular, explained using the example of a semiconductor wafer. Thesurface of a wafer should be free from conchoidal fractures after asawing process, and be free from particles, and linking units such asfuses should be intact. Devices or methods that can be integrated intothe organization of the fabrication should be applied in the appropriatetesting of the surface in order to detect defects.

Depending on the size of a test object, and depending on the requiredresolution, a manual visual inspection with the aid of obliquelyincident light and of a test object that executes wobbling movementscan, for example, take place between individual method steps in thefabrication. Furthermore, an automatic inspection can be performed withthe aid of a line scan camera by scanning once over the entire object.The number of pixels in the line and the width of the image dictate thepixel resolution per line unit. This is typically 40 μm.

Automatic inspection in the case of a conventional scanning operation isalso known. Here, the surface of the test object is scanned with the aidof a two-dimensionally resolving camera. Variations in the illuminationpermit resolutions at different levels. Inspection with the aid of laserscanning on the basis of different principles is a further method fromthe related art. In this case, the laser beam mostly scans the object,object and laser beam moving at high speed relative to one another.

The method from the related art that is most promising for the automaticinspection of wafer surfaces is automatic inspection with the aid of atwo-dimensionally resolving camera. However, manual visual inspectionalso continues to be applied.

SUMMARY OF THE INVENTION

It is one possible object of the invention to make available asufficiently fast and reliable system for the fabrication, and a methodfor testing surfaces on test objects, it being possible at the same timeto achieve a sufficiently high resolution.

One aspect of the invention is based on the finding that it is possibleto inspect surfaces of a test object by using a system in a measuringhead comprising a one-dimensionally resolving line scan camera, anoptical system and a lighting unit in conjunction with a highlyresolving positioning system, the positioning system moving the testobject relative to the measuring head with the line scan camera. It isparticularly provided in that case that the camera, which produces animage strip stepwise or continuously, is moved over the entire surfaceof the test object or of the prescribed regions thereof. The image lineor the individual pixels of the image line can be read out serially orin parallel. To inspect the surface of the test object, the measuringhead and test object are moved relative to one another in a meanderingfashion so to give the selected surface is completely scanned. The imagestrips thus produced are evaluated online (in real time), or assembledto form an overall image and evaluated subsequently.

It is particularly advantageous to use a microscope as the opticalsystem. In conventional illumination, this permits defects to be foundin the surface of an object to be represented with better resolution.

A measuring head or a line scan camera with an objective or a microscopeis moved relative to the surface of an object in order to fully examinea surface of the latter. This is performed by the lateral movement of anobject that is fastened on a positioning system. With regard to theresolution, required for the inspection, on the image side is fulfilledby this positioning system via an appropriately fine rastering ordetermination of position in the lateral movement of the object. Arelative path of movement between the line scan camera and object canpreferably be meandering, spiral or circular. The spiral or circularrelative movement is best suited to circular wafers. For the purpose ofevaluating the overall object scene, the image strips detectedsequentially and picked up by the line scan camera are parts of anoverall image that is assembled in an evaluation unit. The evaluation ofthis image with regard to surface defects that occur can be performedsimultaneously or later with the aid of prescribed categorizationfeatures.

It is advantageous to have different types of illumination with regardto light field, dark field, or else transmitted light in order tomanipulate surfaces reflecting in different ways, in particular speculasurfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 shows a line scan camera that sequentially picks up amultiplicity of lines 5 on the object surface in the direction ofmovement 2 relative to the image strips 1 to be picked up overall,

FIG. 2 shows the structure of the overall image that is produced bymeandering movement over the object surface in accordance with FIG. 1,and subsequent assembly of the image strips to form an overall image,and

FIG. 3 shows a currently picked up line 5 that sweeps over a ring on anobject surface by being picked up sequentially.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

A substantial advantage in the use of a line scan camera for inspectingobject surfaces resides in simplified illumination. In addition, it ispossible in conjunction with the same resolution to analyze largersamples than in the case of using a two-dimensionally resolving camera.Larger strip widths are therefore available in order to accomplish ananalysis of the surface of an object. For example, a segment with1000×1000 pixels in conjunction with 8 k pixels×resolution per pixel isa possibility using a two-dimensionally resolving camera. In the case ofa one-dimensionally resolving line scan camera that is movedtransversely to the longitudinal extent of the line in conjunction witha highly resolving positioning system for an object, a resolution of 1μm is possible, when a line with 8000 (8 k) pixels, for example, is usedand the relative movement between the object and camera is 10 mm/sec. Afurther advantage relates to the continuous imaging. By contrasttherewith, a two-dimensional camera requires a plurality of individualimages overlapping in the edge region for the purpose of mutualorientation.

The system and method can be used, in particular, for all types ofoptical inspection on wafer surfaces.

Use is made of a one-dimensionally resolving line scan camera having,for example, 4000 or 8000 pixels, a highly accurate positioning table,which can be positioned or sets an object position, a lighting unit, anoptical system, in particular a microscope, that is positioned betweenthe camera and object, and an image processor for storing the datapicked up and for analyzing these image data. The analysis of the imagedata can be performed simultaneously (real time) or subsequently. FIG. 1shows how the surface to be inspected is picked up with the aid of theline scan camera 4 in a meandering fashion corresponding to theprescribed direction of movement 2. This can also be performed incircular fashion in accordance with FIG. 3, movement being performedover a plurality of circles of different radii. The object is movedprecisely under the line scan camera with the aid of a positioningdevice during the scanning operation.

FIG. 1 shows that the camera 4 has a microscope, for example, arrangedupstream and is currently picking up a line 5, and that an image strip 1is picked up by aligning a multiplicity of lines 5 stepwise orcontinuously. In this case, the length of a line 5 (longitudinal extent)extends in the x-direction, and the width of a line 5 extends in they-direction (0 direction of movement). An image strip 1 is thereforereduced overall by aligning the lines 5.

FIG. 2 shows a surface that has already been scanned in at the start ofthe pick-up operation. The image strip 1 that is to be detected has beendefined previously. The path of movement, that is to say the directionof movement 2, in which the image strips 1 are picked up is likewisefixed. The overall predetermined surface of an object can be detectedthereby.

During a scanning operation, the image strip position of the start ofthe strip is linked to the position of the positioning system. A mutualrelative assignment of the image strips 1 is detected thereby. A furtherapplication provides that only half of a wafer is scanned in, and thewafer support (chuck), on which the wafer rests, is rotated by 180°before a second scanning operation. The entire wafer is thereby scannedoverall with the aid of single-axis positioning.

Illumination of the object surface can be a permanent illumination, butit is also possible to use flashes for specific requirements. The objectis generally set rotating in order to constitute circular or annularscanning. The decision as to which type of illumination, bright field,dark field or transmitted light is used depends on the achievablecontrast on the object surface. For example, if the aim duringinspection is a resolution of 1 μm in conjunction with the imaging ofthe object on a semiconductor chip, the system could, for example, beoperated at 10 MHz corresponding to 10 megapixels per second.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

1. A system for inspecting object surfaces, comprising: a onedimensionally resolving semiconductor line camera having an objectiveand having; a microscope in the objective, which camera is movablerelative to said objective surface, a lighting unit, and a highresolution positioning system bearing the object to continuously movethe object relative to the semiconductor camera at least in terms of twodimensions in order for the semiconductor image at least one imagestrip.
 2. The method as claimed in claim 1, wherein the lighting unit isa changeable lighting unit to vary the lighting characteristics whileimaging the object.
 3. A method for inspecting object surfaces,comprising: positioning an object on a high resolution positioningdevice; focusing a semiconductor line scan camera with one dimensionalresolution at a particular point on the position device, through amicroscope, the camera having an objective and the microscope being partof the objective of said camera; moving the positioning device so as toselectly move different portions of the object into focus for thesemiconductor camera; and imaging the object while moving thepositioning device.
 4. The method as claimed in claim 3, wherein imagestrips are sequentially imaged by the semiconductor camera and evaluatedin real time.
 5. The method as claimed in claim 3, wherein image stripsare sequentially imaged by the semiconductor camera and assembled in anevaluation unit and evaluated subsequently.
 6. The method as claimed inclaim 3, wherein lighting is performed by transmitting light on theobject, in a bright field.
 7. The method as claimed in claim 3, whereinlighting is performed by transmitting light on the object, in a darkfield.
 8. The method as claimed in claim 3, wherein the surface of asemiconductor wafer is inspected.
 9. The method as claimed in claim 3,wherein the positioning system is triggered by camera exposure.
 10. Themethod as claimed in claim 3, wherein the object is illuminated usingflashes of light.
 11. The method as claimed in claim 3, wherein theobject is illuminated using at least one of transmitted illumination andincident illumination, and the incident illumination is executed as darkfield or light field illumination.
 12. The method as claimed in claim 3,wherein light emitting diodes are used for the transmitted illumination.13. The method as claimed in claim 3, wherein the positioning device ismoved along a meandering path.
 14. The method as claimed in claim 3,wherein the positioning device is moved along a rectilinear path. 15.The method as claimed in claim 3, wherein the positioning device ismoved along a spiral path.
 16. The method as claimed in claim 3, whereinthe entire object is imaged while moving the positioning device.
 17. Themethod as claimed in claim 3, further comprising changing theillumination on the object while imaging the object.
 18. The method asclaimed in claim 8, wherein the positioning system is triggered bycamera exposure.
 19. The method as claimed in claim 18, wherein theobject is illuminated using flashes of light.
 20. The method as claimedin claim 19, wherein light emitting diodes are used for illumination.